33 ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , R IO DE JANEIRO 2013
T HE A STROPARTICLE P HYSICS C ONFERENCE

A Proposal of a Multi Dire tional Neutron Teles ope for Observations of Gala ti Cosmi Rays

H.KOJIMA1 , A.O SHIMA2 , S.S HIBATA2 , Y.H AYASHI3 , H.A NTIA4 , S.D UGAD4 , T.F UJII3 , S.K.G UPTA4 ,S.K AWAKAMI3 , M.M INAMINO3 , P.K.M OHANTY4 , I.M ORISHITA5 , T.NAKAMURA6 , T.N ONAKA7 , S.O GIO3 ,H.TAKAMARU2 , H.TANAKA3 , K.TANAKA8 , N.I TO3 , A.JAIN4 , T.M ATSUYAMA3 , B.R AO4 , K.YAMAZAKI3 ,N.YOSHIDA2 FOR THE ICRC C OLLABORATION .
1 Ai hi Institute of Te hnology
2 Chubu University
3 Osaka City University
4 Tata Institute of Fundamental Resear h
5 Asahi University
6 Ko hi University
8 Hiroshima City University
7 University of Tokyo
shibatais . hubu.a .jp

Abstra t: For the purpose of observing solar neutrons produ ed at solar ares, the solar neutron teles ope has
been using, whi h has veto dete tors to reje t the in iden e of harged parti les and at hes the re oil proton
from produ tion layers as the signal of neutron in iden e. In this paper, by taking advantage of the dete tor todis riminate the dire tions of in ident neutron, we propose a new appli ation to investigate the time variations
and anisotropies of gala ti osmi rays.
Keywords:

1neutron, teles ope, anisotropies of gala ti

Introdu tion

Both intensity variations and anisotropies of gala ti osmi rays ould be organized into two different types su h
as stable variations and temporal variations. These are believed to be related to the most important mystery of theorigin of gala ti osmi rays and their propagation. The
stable variations in gala ti osmi rays are formed by thestru ture of the magneti eld of the inner solar system
whi h ould be modulated by the solar a tivity and solar
wind. The temporal variations, on the other hand, are related to sudden phenomena like Forbush de reases ausedby solar ares.
1.1

Stable variations ofosmi

rays

Solar wind plasma and the solar magneti eld form a spiral stru ture of the magneti eld of the inner solar system
whi h is known as Parker's spiral. But the spiral stru ture
ould be varying in time moderately by the variation of solar wind velo ity. The propagation or transport of gala ti osmi rays in su h a magneti eld an be des ribedby the Parker's theory of diffusion and onvention of osmi rays. A u tuation of su h an interplanetary magneti eld strength is thought to affe t the propagation of gala ti osmi rays, be ause of an intera tion between magneti eld and ele tri harges of osmi rays.
1.2

Temporal variations ofosmi

rays

Solar ares liberate an enormous magneti energy at thesurfa e of the sun and provide the energy to a sho k wave
whi h would be reated in the solar wind plasma. Thesho k wave auses several ele tromagneti phenomena in
the interplanetary spa e in luding the earth's neighborhood spa e. The turbulen es of su h an ele tromagneti

osmi rays, GRAPES-3.

eld leads to a sudden variation of the gala ti osmi rayintensity, like Forbush de reases.
The stable and temporal variations observed in the intensities and anisotropies of gala ti osmi rays, whi hpropagate in the spiral stru ture of interplanetary magneti
eld, are thought to be typi al ele tromagneti phenomena that might happen in the universe. Therefor the detailstudy of both the intensity variations and the anisotropies
of gala ti osmi rays in the inner solar system will leadto a basi understanding of me hanisms of various phenomena aused by astronomi al plasma.
The effe t of the magneti sho k wave a ompanied bysolar wind plasma on the ele tromagneti environment of
the earth is quite serious be ause that our modernized soiety is supported by highly developed ele troni devi es.
The study of gala ti osmi rays in the inner solar system,
espe ially near earth is important for our real life on earth
as a spa e weather.
So far, su h a resear h mentioned above has been arried by using radio wave, visible right, x-ray satellite andparti le rays. However the observables from these measurements are based on the dire t images of a phenomenonon the surfa e of the sun, or some physi al quantities ofsolar wind plasma or of interplanetary magneti eld measured at the earth's neighborhood spa e. Furthermore, observations of atmospheri neutrons indu ed by gala ti
osmi rays at the top of the earth's atmosphere have only
been arried by the equipment without the fun tionality ofmeasuring parti le energy and dire tions.
Here, we propose a new type of teles ope of atmospheri neutrons, whi h will be installed at the site of
GRAPES-3 experiment, to investigate the time variations
and anisotropies of gala ti osmi rays.

Neutron Teles ope

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2ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , R IO DE JANEIRO 2013

GRAPES-3

The experimental system of the GRAPES-3 (Gamma RayAstronomy at PeV EnergyS Phase-3) experiment onsistsof a densely pa ked array of s intillator dete tors and alarge area tra king muon dete tor. The EAS array onsistsof 350 plasti s intillator dete tors , ea h of 1 m2 in area.
These dete tors are deployed with an inter-dete tor separation of only 8m. The array is being operated at Ooty insouth India (11:4Æ N, 76:7ÆE, 2200m altitude).
The 560 m2 GRAPES-3 muon dete tor onsists of 4super-modules, ea h in turn having 4 sub-modules. Ea hsub-module with a sensitive area of 35 m2 onsists of a total of 232 proportional ounters (PRCs) arranged in 4 layers, with alternate layers pla ed in orthogonal dire tions.
Two su essive layers of PRCs are separated by 15 m
thi k on rete. The energy threshold of 1 GeV for verti almuons, has been a hieved by pla ing a total of 15 layersof on rete blo ks (total absorber thi kness 550 g m 2 )above the top layer. The on rete blo ks have been arranged in the shape of an inverted pyramid to provide adequate shielding up to a zenith angle of 45Æ .
3

Design of Neutron Teles ope

For the purpose of dete ting atmospheri neutrons produ ed by high energy osmi rays, the neutron teles ope
has been using, whi h has veto dete tors to reje t the iniden e of harged parti les, dete tion of the re oil proton
from produ tion layers as the signal of neutron in iden e.
In our proposal, the newly designed neutron teles ope andete t re oiled protons whi h might be ki ked out by neutrons in the produ tion layer as a typi al neutron dete tor,and furthermore it an re ord their in ident dire tions also.
We show the s hemati image of the newly designed
neutron teles ope in Figure.1. The teles ope onsists ofseveral layers of the dete tor for a spe i purposes ea h.
Two layers of proportional ounter tubes at the top ofteles ope perform as the veto dete tors reje ting an ele tromagneti omponent and muon omponent of the se ondary parti les. The layers made up of timbers and s intillators, whi h has an area of 9 m2 , are the produ tion layers for onversion of atmospheri neutrons to re oiled protons. The remained 6 layers of proportional ounter tubes
will be operated as a parti le tra king dete tors that anre ord the dire tion of the re oiled protons. Ea h layer of
proportional ounter tubes onsists of 58 tubes and has thearea of 35 m2 as a sensitive area. They are arranged withalternate layers pla ed in orthogonal dire tions to identify
the parti le tra ks.
4Simulation

This work shows the results of Monte Carlo simulations toquantify the hara teristi s of atmospheri neutrons due tohigh energy osmi rays of various primary energies. thehara teristi s of atmospheri neutrons at an observationlevel have been al ulated to estimate the possible observables derived from the new neutron teles ope. COSIKA(COsmi Ray Simulations for KAskade), whi h is widelyused by high energy osmi ray experiments around theworld, was used for this purpose. The CORSIKA was setup with the parameter of QGSJII for high energy hadroni
intera tion and with GEISHA for low energy. The energy
range of primary parti les was set from 1GeV to 1TeV be-ause that the median rigidity of primary protons at thegeomagneti latitude at the GRAPES-3 site is about 100
GV. Sin e the CORSIKA doesn't tra e hadrons with energies less than 50MeV in the simulation, the atmospheri
neutrons also were able to be followed up to the energyof 50MeV. The zenith angle of in ident primaries were set
uniformly between 0 and 50 degrees.
5Results

We've simulated development of the se ondary parti lesin the atmosphere with primary protons of energy from 1
GeV to 1 TeV that have been divided into 12 points at
equal interval in logarithmi s ale. To quantify the primary
angular information of whi h the se ondary neutrons ankeep through the development in the atmosphere, we took
the value of an opening angle between ve tor of a neutronand of the primary proton.
Be ause that the se ondary neutrons have undergoneseveral s attering in their passages, the distribution ofopening angle of se ondary neutrons has broad distribution with the width of about 10 degrees. The opening angle distribution of se ondary neutrons indu ed by the primary with an energy of 23.1 GeV in the Figure.2. In thisgure, we set a threshold energy for se ondary neutrons at1 GeV and 2 GeV respe tively, and found that there is nosigni ant differen e in the opening angle even though theneutron energies are different. As in the Figure.3, the distribution of opening angle of the primary energy of 1 TeV
is similar to that of Figure.2.

Figure 2: Opening angle distribution of the se ondary neutrons relative to the in ident dire tion of primary protons.
The energy of the primary is xed at 23.1GeV

6Summary

The median rigidity of primary protons observed by
GRAPES-3 muon teles ope is around 100 GV with the assumption that the strength of interplanetary magneti eld
is about 5 nT. On the other hand, the median rigidity ofprimary protons observed by the neutron teles ope, whi hNeutron Teles ope

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ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , R IO DE JANEIRO 2013

: The s hemati image of the newly designed neutron teles ope.

Figure 1

resolving the spa e of the neutron teles ope is almost thesame as that of the muon teles ope.
As we mentioned above, neutron omponents and muon
omponents of se ondary parti les an ree t the different
energy region of the primary osmi rays. This allows us
to prove the stru ture of Inter magneti eld at different
energy region.
A knowledgment:This work is supported by the grants-inaid of the Solar-Terrestrial Environment Laboratory of Nagoya
University and the Institute for Cosmi Ray Resear h of University of Tokyo.

Referen es[1℄ H.Kojima, et al. Pro .32nd Int.Conf., Beijng (2011) SHi r 0983 Web[2℄ H.Kojima, et al. Pro .31st Int.Conf.,Lodz,(2009) Vol.
1(SH),p 359-362[3℄ P.Subramanian, et al. Astronomy Astrophysi s 494, 1107 窶1118 (2009)

Figure 3: Opening angle distribution of the se ondary neutrons relative to the in ident dire tion of primary protons.
The energy of the primary is xed at 1000GeV

has been introdu ed in this paper, is about 30 GV at the siteof GRAPES-3, India. The Larmor radius of primary protons at the median rigidity in the interplanetary magneti
eld observed by both the teles ope has been estimated to
be about 0.5 AU and 0.15 AU respe tively. It implies that
we an explore the earth neighborhood spa e with diffrents ale of spa e by observing the primary protons of different rigidity. However, in this work, we've found that these ondary neutrons, whi h are produ ed by the high energy osmi rays or produ ed by the se ondary nu lei inthe atmosphere, an have large u tuations in an angular information that is three times larger than the muon
teles ope. The angular resolution of the muon teles ope
is approximately 10 degrees. Therefore the apability of